The present invention relates to a control circuit for video endoscope.
Improved video endoscopes have recently been developed with the progress of solid-state pickup elements. In these endoscopes, a solid-state pickup element, such as a charge-coupled device (CCD), is contained in the distal end of the endoscope, and used to pick up an image of the interior of the body cavity. The image from the pickup element is transmitted, through a signal line in the endoscope, to an outside display unit, whereupon it is displayed.
Conventionally, a light source of an endoscope is provided as a light source unit, independent of the body of the endoscope, and the endoscope is connected to the unit by means of a universal cord, which extends from the endoscope body. An illumination light from the light source unit is transmitted through a light guide fiber in the universal cord and the endoscope body, and is then applied to an object.
Video endoscopes of this type require a driver circuit for generating clock pulses, used to drive the solid-state pickup element, a video processing circuit for video-processing an image signal from the pickup element, and other circuits. Usually, these circuit units are disposed in the light source unit. The clock pulses for the pickup element and image signals from the element are transferred between the light source unit and the pickup element, by means of a signal line in the universal cord.
In general, endoscopes are available with various lengths and diameters, depending on the region into which the endoscope is to be inserted. Meanwhile, in order to commonly use the aforesaid circuit units for the various endoscopes, the same solid-state pickup element is used in all types of video endoscopes. If the length of an endoscope is different from that of another, however, the length of the signal line, extending from the circuit units in the light source unit to the distal end of the endoscope, varies correspondingly, thus resulting in the following awkward situations.
If the signal line is lengthened, the waveform of the clock pulses, supplied from the driver circuit to the solid-state pickup element, is deteriorated and ceases to be an exactly square one. Accordingly, the pickup element cannot be driven correctly. Further, the transmission of the image signals, from the element to the video processing circuit, is delayed. The image signals delivered from the pickup element are intermittent picture-element signal pulses. Therefore, the video processing circuit must first convert them into continuous image signals by clamping them, or by sampling and holding them. Such a process must be synchronized with the generation timing of the drive clock pulses. If the transmission of the image signals is delayed, the synchronism cannot be maintained, so that extra data, not including the picture-element information, will be clamped, or sampled and held. Thus, accurate image signals cannot be obtained. Moreover, the influences of the delay of transmission on the signal line vary according to the endoscope length. Conventionally, therefore, two or more different types of video endoscopes, with different lengths, cannot be connected to a single light source unit.
With use of these prior art video endoscopes, furthermore, diagnoses are made frequently on the basis of the color of a displayed image of the affected part, rather than its shape. Accordingly, the color-reproducibility of the display unit should be considerably accurate, and its color adjustment must be performed carefully. In a conventional method of color adjustment, a color chart is picked up in advance, and the image color is adjusted in accordance with the chart, displayed on a screen. However, the distal end portion of the video endoscopes has a diameter of a little more than ten millimeters, and the angle of view is very wide. Therefore, the photographing of the color chart is a delicate work, so that the color adjustment cannot be performed with speed.